1. Field of the Invention
The present invention relates to an optical amplifier using an optical amplification medium doped with a rare earth element, and in particular, to an optical amplifier to improve efficiency of optical amplification for a wavelength band that differs from a typical amplification band.
2. Description of the Related Art
An optical amplifier is a device that amplifies input light as it is without converting it into electricity. In an optical amplifier, generally, a wavelength of light to be amplified is limited depending on an applied optical amplification medium. With the increase of the capacity of optical transmission system in recent years, optical wavelength division multiplex transmission, in which signal light of different wavelengths is multiplexed together, has become available. Since the transmission capacity of an optical wavelength division multiplex transmission system depends on the wavelength band in which the optical amplification produces a gain, it is necessary to extend the wavelength band that includes the gain of the optical amplifier. Therefore, it would be desirable to realize an optical amplifier that amplifies a wavelength band that has been considered difficult to amplify previously. As a candidate for a new transmission wavelength band of the optical transmission system, for example, a wavelength band called S-band of from 1480 to 1530 nm is under consideration and the optical amplifier corresponding to S-band is researched actively.
Optical amplifiers for S-band, such as a gain-shifted thulium (Tm)-doped optical fiber amplifiers (GS-TDFA), a-discrete Raman amplifiers and the like are known. GS-TDFA, however, does not have sufficient reliability in fluoride optical fiber, while the Raman amplifier produces a nonlinear effect when high power at the output is suppressed. In such circumstances, one of the promising candidates for technology that enables a practical optical amplifier for S-band is the shift of an amplification band of an erbium doped optical fiber amplifier (EDFA) to S-band.
The EDFA has already been commercially available as an optical amplifier for C-band (1530-1560 nm) and L-band (1570-1610 nm). FIG. 21 exemplarily shows population inversion rate dependence of a relative gain coefficient of a typical erbium doped optical fiber (EDF). Note, the relative gain coefficient in the ordinate is expressed by standardizing a gain per unit length of the EDF. The population inversion rate represents a pumping condition of the EDF, which is a three level system in this case, and can be expressed by a relation shown in the following equation (1).population inversion rate=Er ion density of upper level/total Er ion density  (1)
In FIG. 21, in a typical EDFA, the EDF is pumped at the population inversion rate in which a gain wavelength characteristic of an amplification band is substantially flat. More specifically, the population inversion rate in a conventional C-band EDFA is about 0.7, and on the other hand, the population inversion rate in an L-band EDFA is about 0.4. Focusing attention on the S-band corresponding to an area enclosed by oblique lines in FIG. 21, it can be seen that the relative gain coefficient becomes positive when the population gain coefficient is 0.7 or more.
However, in the case where the EDFA is applied to the amplification of S-band light as described above, as opposed to the conventional operating condition for C-band or L-band, a problem is caused in that: (a) the gain coefficient becomes maximum at a wavelength outside the amplification band (in the vicinity of 1530 nm), and (b) a population inversion rate does not exist in which the gain wavelength characteristic in the amplification band becomes substantially flat. In particular, with regard to problem (a), ASE (amplified spontaneous emission) light generated in the vicinity of 1530 nm in the EDF is increased, which would be a factor to reduce efficiency. Due to these problems described above, it was practically difficult to increase the gain of S-band in the EDFA.
Therefore, as a measure for solving the problems of the S-band EDFA as described above, the applicant of the present invention proposed a constitution in which the ASE light in the vicinity of 1530 nm is suppressed by inserting optical filters between the EDFs that were connected in multi-stages as shown in FIG. 22, for example (Japanese Unexamined Patent Publication No. 2001-313433 and Japanese Patent Application 2001-252165).
In the constitutional example in FIG. 22, signal light input from an optical transmission path is input via an optical isolator for preventing reflected light and a WDM coupler to the EDFs and optical filters connected in multi-stages. At this time, pumping light from a pumping light source is supplied via the WDM coupler to the EDF serving as an optical amplification medium. The signal light input to the EDF is amplified due to a stimulated emission phenomenon of by Er ion. The ASE light in the vicinity of 1530 nm generated at this time is suppressed by the optical filter having a transmission wavelength characteristic as shown in FIG. 23. Further, a gain deviation occurring at the time of amplification is flattened by the optical filter described above. The signal light having passed through the EDF and optical filters of each stage is output via a WDM coupler and an optical isolator at an output side to the optical transmission path.
However, in the S-band EDFA as described above, although the amplification and growth of the ASE light of a band other than S-band can be suppressed by inserting the optical filters between the multi-staged EDFs, it is difficult to suppress the growth of the ASE light perfectly. Therefore, there is caused a problem in that efficiency of optical amplification is low in comparison with other EDFAs corresponding to other bands.
Further, in order to improve efficiency of the S-band EDFA while securing a required gain, it can be contemplated to keep the ASE light power input to the EDF at each stage as low as possible by increasing the number of the optical filters inserted between the stages, for example, but in this case, there is also caused a problem in that the constitution would be more complicated than that in the conventional EDFA.